Generators, which may provide electrical power in a power system, are normally sensitive to disturbances in the operating conditions of the power system to which they are connected.
A disturbance or a fault condition in the power system may give rise to an out-of-step condition, i.e. the loss of synchronization between different areas within a power system, or alternatively loss of synchronization between e.g. two interconnected power systems.
During an out-of-step condition, high peak currents, off-frequency operation, and pulsating torques due to the rotor angle varying between 0 and 2π radians may cause electrical and mechanical stress to the generator. By rotor angle is meant the angle between the magnetic axis' of the rotor and the stator of the generator.
When a synchronous generator loses synchronization, i.e. when an out-of-step fault occurs, the rotor flux loses grip of the stator flux, whereby the rotor angle will change. As a result, the generator loses torque. The rotor speed will then increase, as will the generated currents. A pulsating torque may be a result of the variation of the rotor angle. In this situation, it is normally desirable to disconnect the generator from the power system so as to prevent damage to the generator.
Typically, circuit breakers are provided at the generator end of the power system. Circuit breakers, at least one provided for each electrical phase, serve to disconnect the generator from the power system when a fault condition occurs so as to break the current path in the affected part of the power system. In order to function properly, the circuit breakers should be exposed to as little electrical and mechanical stress as possible.
In current solutions for reducing the stress on the circuit breakers, as well as on the generator, the generator is disconnected from the power system after a predetermined number of torque pulses after an out-of-step condition has been detected. The generator is hence subject to severe conditions until the circuit breakers disconnect the generator from the power system. A drawback with the current solution is that the circuit breakers as well as the generator are exposed to high stress prior to disconnecting the generator from the power system.